1. Field of the Invention
The present invention relates to an inkjet printing apparatus for controlling driving of a printhead and a printhead control method of the apparatus.
2. Description of the Related Art
In recent years, inkjet printers are now widespread and common in home use due to a drop in price. They are becoming more complex to support, for example, a direct print function from a portable medium such as an IrDA or Compact Flash® as well as a copy function. The inkjet printers are currently used to print not only simple documents but also images such as photos. Under these circumstances, higher image quality is demanded.
One of the factors that greatly influence the image quality of an inkjet printer is the stability of ink amounts discharged from nozzles provided on the inkjet printhead. To improve photo image quality, smooth gradation printing is necessary. This is attained by distributing the number of discharged droplets of color inks of yellow, magenta, cyan, and the like to smoothly print gradation. Gradation is thus expressed by the number of discharged droplets. Hence, if the discharge amounts have errors, smooth gradation cannot be expressed.
One of inkjet printhead schemes heats a heater provided in each nozzle to generate bubbles and discharges ink from the nozzle by the force of the bubbles. In this scheme, the amount of heat to be applied to the heater is controlled, thereby controlling the bubble generation amount and consequently controlling the discharge amount. However, during continuous printing, heat storage around the nozzles, heat storage of inks in the ink tanks, a temperature rise in the printing apparatus, and the like influence the discharge amount. For this reason, the discharge amount varies even when a predetermined amount of heat is continuously applied to the heater. To avoid this, it is necessary to detect the temperature near the inkjet printhead and control the amount of heat to be applied to the heater in consideration of the temperature information.
As an arrangement for detecting the temperature near the inkjet printhead, a temperature sensor such as a diode sensor is sometimes provided in the printhead. However, the current to make the sensor function is much smaller than the current to drive the printhead and is therefore readily affected by noise. Additionally, print data transfer to the printhead is done at several tens of MHz in order to increase the speed of the printing apparatus. The influence of noise from such a driving signal is also large.
To avoid the influence of noise between the driving signals of the temperature sensor and the printhead, a measure is conventionally taken on a substrate pattern or using a bypass capacitor. However, this method poses the following problem.
The accuracy of temperature detection by a diode sensor during a printing operation is poor because of the influence of noise generated by printhead driving. Hence, control needs to be performed while ensuring a large margin. This increases the time and frequency of pauses to lower the printhead temperature.
On a substrate pattern, a measure is taken by surrounding the noise source by a ground. Alternatively, a flexible substrate without any high-rate signal arranged adjacent to a sensor signal is used, or a plurality of flexible substrates are stacked without arranging any high-rate signal at the same position of opposing flexible substrates. However, these methods impose restrictions on the pattern design.
The waveform of, for example, a clock signal used to transfer print data to the printhead often exhibits a fall time shorter than a rise time and largely affects the diode sensor signal in falling. For this reason, if a measure is taken using only a bypass capacitor, the noise smoothing result is localized to the positive or negative side. That is, the influence of noise cannot be completely eliminated.
An arrangement for avoiding the influence of noise of printhead driving detects a period where the printhead is not driven and detects the temperature during that period. According to the description of Japanese Patent Laid-Open No. 7-266564, the disable period of a pulse signal which controls heating for ink discharge is detected, and temperature data is sampled during the detected period. This described method is said to be able to detect temperature accurately because temperature data is sampled during the period without the noise generation source.
In this proposal, however, the disable period of the pulse signal for printhead control is simply detected. If the temporal density of driving signals rises along with an increase in the speed of the printing apparatus, or the number of driving signals increases for more complex driving, it may be impossible to obtain a disable period sufficient for temperature data sampling. Additionally, since the carriage position at the time of temperature detection is not detected, it is impossible to manage the appropriate carriage position to be reflected on print pulse control.
In another arrangement, not only the printhead driving signal but also another signal for motor driving, for example, is regarded as a noise source. According to the description of Japanese Patent Laid-Open No. 9-193395, a temperature information signal is received only when the carriage motor for moving the printhead or carriage or the conveyance motor for conveying a printing medium is not driven. This method is stated to be able to accurately measure the temperature without any influence of noise from not only the printhead but also other noise sources.
However, since the disable period of a portion serving as a noise source is detected simply, it may be impossible to obtain a disable period sufficient for temperature data sampling, as in the above-described arrangement. Japanese Patent Laid-Open No. 9-193395 describes a carriage motor and a conveyance motor for conveying a printing medium. In a recent printing apparatus, the carriage motor and conveyance motor sometimes operate simultaneously at the time of switching to speed up the operation. In such a case, there is no noise source disable period, and temperature control of the printing apparatus is impossible. In Japanese Patent Laid-Open No. 9-193395 as well, since the carriage position at the time of temperature detection is not detected, it is impossible to manage the appropriate carriage position to be reflected on print pulse control.
An arrangement also exists which obtains, in an inkjet printhead using piezoelectric elements, a temperature from the electrostatic capacitance of a piezoelectric element for discharge which is provided in each ink nozzle. Japanese Patent No. 3948939 states that since a piezoelectric element accumulates charge by driving pulses applied for ink discharge, a change in the charge accumulation amount caused by the temperature is detected, thereby detecting the temperature of each nozzle.
However, Japanese Patent No. 3948939 does not mention noise during temperature detection at all. In addition, since the carriage position at the time of temperature detection is not detected, the appropriate position to be reflected on print pulse control cannot be managed, as in the above-described techniques.
Conventionally, a driving signal disable period having a time width necessary for temperature data sampling cannot be obtained in some cases. That is, it is sometimes impossible to obtain temperature data at a necessary timing. In addition, since the position of the inkjet printhead during temperature detection is unknown, it may be impossible to switch the driving pulse at an appropriate timing.